The present invention relates in general to laser treatment of biological tissues and structures including blood vessels. The invention relates in particular to methods of detecting onset of coagulation in the blood vessels.
In recent years, use of lasers for treatment of dermatological conditions and other conditions involving blood vessels has rapidly gained acceptance by the medical community. By way of example, lasers delivering light in the green and yellow regions of the visible spectrum are now effectively used to treat vascular disorders. The green and yellow wavelengths between about 530 and 590 nanometers (nm) are particularly favored because absorption of these wavelengths in blood hemoglobin is significantly higher than in the skin pigment melanin. This allows the hemoglobin to be selectively targeted by the radiation in a treatment often referred to as selective photothermolysis. In such a treatment, blood in targeted vessels is coagulated by heat generated when the green or yellow light is absorbed in the blood hemoglobin. This results in necrosis of the vessels in the treated area. The coagulated or necrotized vessels are eventually reabsorbed by the body and replaced with scar tissue.
One problem encountered in applying laser treatment is determining a light dosage sufficient to effect the required coagulation. This can vary from patient to patient depending on factors such as melanin content of the skin (skin color), the size (diameter) of blood vessels being treated and the depth of the blood vessels in the skin. Extending dosage beyond that required to effect coagulation can lead to complications including scarring, scabbing, edema and epidermal damage or extravasation of red blood cells from a vessel being treated. Extravasation can lead to formation of progressive purpura which will eventually disappear, but which are generally cosmetically unacceptable to patients under most circumstances.
An increase in reflection and scatter of treatment radiation has been observed in connection with lesion formation in retinal photocoagulation. Skin, however, is highly scattering of yellow/green radiation. Accordingly any increase in reflection or scatter resulting from coagulation of blood in a blood vessel in or under the skin will be difficult to discriminate from background scatter and reflection. Accordingly there is need for a method of detecting the onset of coagulation which effectively discriminates against background scatter and reflection of treatment radiation.
The present invention is directed to a method of detecting onset of coagulation of blood in a blood vessel irradiated by electromagnetic radiation having a selected treatment wavelength. The method comprises directing electromagnetic radiation having a selected monitoring wavelength to the blood vessel, the monitoring wavelength being different from the treatment wavelength. A portion of the monitoring radiation remitted from the blood vessel is detected. One of an increase and decrease in the detected remitted monitoring radiation is interpreted as an indication of the onset of coagulation of blood in the blood vessel.
The method of the present invention is based on detecting a change in absorptivity of blood irradiated by the treatment wavelength characteristic of conversion of oxyhemoglobin and de-oxyhemoglobin in the normal blood to methemoglobin. Depending on the wavelength the absorptivity of methemoglobin can be higher or lower than the absorptivity of normal blood. By way of example, in a wavelength range between about 525 and 580, methemoglobin has a lower absorptivity than normal blood. In wavelength ranges between about 610 and 660 nm and between about 800 and 1000 nm, methemoglobin has a higher absorptivity than normal blood.
In one preferred embodiment of the inventive method, the monitoring wavelength is selected such that the absorptivity thereof in normal blood is greater than or equal to about two times the absorptivity thereof in methemoglobin, and an increase in the detected remitted monitoring radiation is interpreted as an indication of the onset of coagulation of blood in the blood vessel. In another preferred embodiment of the inventive method, the monitoring wavelength is selected such that the absorptivity thereof in normal blood is less than or equal to about one-half the absorptivity thereof in methemoglobin, and a decrease in the detected remitted monitoring radiation is interpreted as an indication of the onset of coagulation of blood in the blood vessel.
One application of the method of the present invention is in dermal treatments wherein the wavelength of treatment radiation is in a range between about 510 and 585 nm, for example, the 532 nm radiation of a frequency-doubled Nd:YAG laser. Here, a preferred monitoring wavelength is selected in a range between about 610 and 660 nm.